Method and MTC device for performing random access procedure for coverage enhancement

ABSTRACT

A disclosure of the present specification provides a method for performing a random access procedure for coverage enhancement. The method may comprise the steps of: transmitting a random access preamble repetitively to a certain cell on the basis of a predetermined repetition level; when a random access response has not received within a random access response window, reconfiguring the repetition level; and retransmitting the random access preamble repetitively on the basis of the reconfigured repetition level.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 15/038,528, filed on May 23, 2016, which is the National Phaseof PCT International Application No. PCT/KR2014/011612 on Dec. 1, 2014,which claims the benefit under 35 U.S.C. § 119(e) to U.S. ProvisionalApplication No. 61/912,030, filed on Dec. 5, 2013, and to U.S.Provisional Application No. 61/930,465, filed on Jan. 22, 2014, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication.

Related Art

3^(rd) generation partnership project (3GPP) long term evolution (LTE)evolved from a universal mobile telecommunications system (UMTS) isintroduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequencydivision multiple access (OFDMA) in a downlink, and uses singlecarrier-frequency division multiple access (SC-FDMA) in an uplink. The3GPP LTE employs multiple input multiple output (MIMO) having up to fourantennas. In recent years, there is an ongoing discussion on 3GPPLTE-advanced (LTE-A) evolved from the 3GPP LTE.

As disclosed in 3GPP TS 36.211 V10.4.0 (2011-12) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 10)”, 3GPP LTE/LTE-A may divide the physical channel into adownlink channel, i.e., a physical downlink shared channel (PDSCH) and aphysical downlink control channel (PDCCH), and an uplink channel, i.e.,a physical uplink shared channel (PUSCH) and a physical uplink controlchannel (PUCCH).

Meanwhile, in recent years, research into communication between devicesor the device and a server without human interaction, that is, withouthuman intervention, that is, machine-type communication (MTC) has beenactively conducted. The MTC represents a concept in which not a terminalused by human but a machine performs communication by using the existingwireless communication network.

Since MTC has a feature different from communication of a normal UE, aservice optimized to MTC may differ from a service optimized tohuman-to-human communication. In comparison with a current mobilenetwork communication service, MTC can be characterized as a differentmarket scenario, data communication, less costs and efforts, apotentially great number of MTC devices, wide service areas, low trafficfor each MTC device, etc.

Recently, it is considered to extend cell coverage of a BS for an MTCdevice, and various schemes for extending cell coverage are underdiscussion. However, when the cell coverage is extended, if the BStransmits a downlink channel to the MTC device located in the coverageextension region as if transmitting a downlink channel to a normal UE,the MTC device has difficulty in receiving the channel. Likewise, whenthe MTC device located in the coverage extension region transmits anuplink channel to the BS in a usual way, the BS may have difficulty inreceiving the uplink channel. In particular, the BS may have difficultyin receiving a physical random access channel (PRACH) among uplinkchannels due to characteristics thereof.

SUMMARY OF THE INVENTION

Accordingly, the disclosure of the specification has been made in aneffort to solve the problem.

To achieve the foregoing aspect, a random access procedure performingmethod according to a first embodiment of the present invention is amethod of performing a random access procedure for coverage enhancement,which may include repeatedly transmitting a random access preamble to aspecific cell according to a preset repetition level, reconfiguring therepetition level based on a preset mode when no random access response(RAR) is received within an RAR window, and repeatedly retransmittingthe random access preamble according to the reconfigured repetitionlevel.

The preset mode may be a mode of configuring the repetition level to bean initial repetition level, in which the initial repetition level maybe a possible lowest repetition level or a repetition level selectedbased on a measurement by a user equipment (UE) or machine-typecommunication (MTC) device.

The preset mode may be a mode of increasing the preset repetition levelby one.

The preset mode may be a mode of increasing the preset repetition levelby one when preamble transmission power P_(PRACH) is configured UEtransmission power.

The preset mode may be a mode of increasing the preset repetition levelby one when preamble transmission power P_(PRACH) at a time ofperforming power ramping is configured UE transmission power or greater.

The preset mode may be a mode of reconfiguring the repetition levelbased on total PRACH power, in which the total PRACH power may be totalpower for subframes corresponding to the repeatedly transmitted randomaccess preamble.

The preset mode may be a mode of reconfiguring the repetition levelaccording to repetition level information included in a PDCCH order whenthe random access procedure starts according to the PDCCH order.

To achieve the foregoing aspect, a method of performing a random accessprocedure according to a second embodiment of the present invention mayfurther include transmitting message 3 (Msg 3) when an RAR is received,reconfiguring a preset repetition level based on an additional presetmode when no message 4 (Msg 4) is received until a contention resolutiontimer expires, and repeatedly retransmitting the random access preambleaccording to the repetition level reconfigured based on the additionalpreset mode.

The additional preset mode may be a mode of maintaining the presetrepetition level in a case where the received RAR includes a backoffindicator, and increasing the preset repetition level in a case wherethe received RAR includes no backoff indicator.

The additional preset mode may be a mode of configuring the presetrepetition level to be a repetition level corresponding to message 3, inwhich the repetition level corresponding to message 3 may be an initialrepetition level or a repetition level corresponding to a random accesspreamble successfully transmitted through retransmission before message3, and the initial repetition level may be a possible lowest repetitionlevel or a repetition level selected based on a measurement by a UE orMTC device.

The received RAR may include a repetition level field, and theadditional preset mode may be a mode of reconfiguring the presetrepetition level according to the repetition level field.

The additional preset mode may be a mode of selecting and performing oneinvolving smaller total RACH power of power ramping and a change of thepreset repetition level, in which the total PRACH power may be totalpower for subframes corresponding to the repeatedly transmitted randomaccess preamble.

The additional preset mode may be a mode of reconfiguring the presetrepetition level according to repetition level information included in aPDCCH order when the random access procedure starts according to thePDCCH order.

To achieve the foregoing aspect, an MTC device according to anembodiment of the present invention is an MTC device performing a randomaccess procedure for coverage enhancement, which may include atransceiver to repeatedly transmit a random access preamble to aspecific cell according to a preset repetition level, and a processor toreconfigure the repetition level when no RAR is received within an RARwindow and to control the transceiver to repeatedly retransmit therandom access preamble according to the reconfigured repetition level.

The transceiver may transmit message 3 (Msg 3) when an RAR is received,and the processor may reconfigure the preset repetition level based onan additional preset mode when no message 4 (Msg 4) is received until acontention resolution timer expires, and control the transceiver torepeatedly retransmit the random access preamble according to thereconfigured repetition level.

Embodiments of the present invention are provided to solve the foregoingproblems of the conventional technology. Specifically, embodiments ofthe present invention may improve reception performance and decodingperformance of an MTC device located in a coverage extension region ofan eNodeB with respect to the eNodeB, thereby implementing a randomaccess procedure in an efficient and excellent manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication system.

FIG. 2 illustrates a structure of a radio frame according to frequencydivision duplex (FDD) of 3rd generation partnership project (3GPP) longterm evolution (LTE).

FIG. 3 illustrates a structure of a downlink radio frame according totime division duplex (TDD) in 3GPP LTE.

FIG. 4 illustrates an example of a resource grid for one uplink ordownlink slot in 3GPP LTE.

FIG. 5 illustrates a structure of a downlink subframe.

FIG. 6. illustrates a structure of an uplink subframe in 3GPP LTE.

FIG. 7 illustrates an example of comparison between a single carriersystem and a carrier aggregation system.

FIG. 8 illustrates cross-carrier scheduling in a carrier aggregationsystem.

FIG. 9a illustrates a contention-based random access procedure.

FIG. 9b illustrates a non-contention-based random access procedure.

FIG. 10a illustrates an example of machine-type communication (MTC).

FIG. 10b illustrates an example of cell coverage extension for an MTCdevice.

FIG. 11 illustrates a method of transmitting or retransmitting a PRACHaccording to an embodiment of the present invention.

FIG. 12 illustrates a random access procedure in case A according to afirst embodiment of the present invention.

FIG. 13 illustrates a random access procedure in case B according to asecond embodiment of the present invention.

FIG. 14 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, based on 3rd Generation Partnership Project (3GPP) longterm evolution (LTE) or 3GPP LTE-advanced (LTE-A), the present inventionwill be applied. This is just an example, and the present invention maybe applied to various wireless communication systems. Hereinafter, LTEincludes LTE and/or LTE-A.

The technical terms used herein are used to merely describe specificembodiments and should not be construed as limiting the presentinvention. Further, the technical terms used herein should be, unlessdefined otherwise, interpreted as having meanings generally understoodby those skilled in the art but not too broadly or too narrowly.Further, the technical terms used herein, which are determined not toexactly represent the spirit of the invention, should be replaced by orunderstood by such technical terms as being able to be exactlyunderstood by those skilled in the art. Further, the general terms usedherein should be interpreted in the context as defined in thedictionary, but not in an excessively narrowed manner.

The expression of the singular number in the present invention includesthe meaning of the plural number unless the meaning of the singularnumber is definitely different from that of the plural number in thecontext. In the following description, the term ‘include’ or ‘have’ mayrepresent the existence of a feature, a number, a step, an operation, acomponent, a part or the combination thereof described in the presentinvention, and may not exclude the existence or addition of anotherfeature, another number, another step, another operation, anothercomponent, another part or the combination thereof.

The terms ‘first’ and ‘second’ are used for the purpose of explanationabout various components, and the components are not limited to theterms ‘first’ and ‘second’. The terms ‘first’ and ‘second’ are only usedto distinguish one component from another component. For example, afirst component may be named as a second component without deviatingfrom the scope of the present invention.

It will be understood that when an element or layer is referred to asbeing “connected to” or “coupled to” another element or layer, it can bedirectly connected or coupled to the other element or layer orintervening elements or layers may be present. In contrast, when anelement is referred to as being “directly connected to” or “directlycoupled to” another element or layer, there are no intervening elementsor layers present.

Hereinafter, exemplary embodiments of the present invention will bedescribed in greater detail with reference to the accompanying drawings.In describing the present invention, for ease of understanding, the samereference numerals are used to denote the same components throughout thedrawings, and repetitive description on the same components will beomitted. Detailed description on well-known arts which are determined tomake the gist of the invention unclear will be omitted. The accompanyingdrawings are provided to merely make the spirit of the invention readilyunderstood, but not should be intended to be limiting of the invention.It should be understood that the spirit of the invention may be expandedto its modifications, replacements or equivalents in addition to what isshown in the drawings.

As used herein, ‘base station’ generally refers to a fixed station thatcommunicates with a wireless device and may be denoted by other termssuch as eNB (evolved-NodeB), BTS (base transceiver system), or accesspoint.

As used herein, ‘user equipment (UE)’ may be stationary or mobile, andmay be denoted by other terms such as device, wireless device, terminal,MS (mobile station), UT (user terminal), SS (subscriber station), MT(mobile terminal) and etc.

FIG. 1 illustrates a wireless communication system.

As seen with reference to FIG. 1, the wireless communication systemincludes at least one base station (BS) 20. Each base station 20provides a communication service to specific geographical areas(generally, referred to as cells) 20 a, 20 b, and 20 c. The cell can befurther divided into a plurality of areas (sectors).

The UE generally belongs to one cell and the cell to which the UE belongis referred to as a serving cell. A base station that provides thecommunication service to the serving cell is referred to as a servingBS. Since the wireless communication system is a cellular system,another cell that neighbors to the serving cell is present. Another cellwhich neighbors to the serving cell is referred to a neighbor cell. Abase station that provides the communication service to the neighborcell is referred to as a neighbor BS. The serving cell and the neighborcell are relatively decided based on the UE.

Hereinafter, a downlink means communication from the base station 20 tothe UE1 10 and an uplink means communication from the UE 10 to the basestation 20. In the downlink, a transmitter may be a part of the basestation 20 and a receiver may be a part of the UE 10. In the uplink, thetransmitter may be a part of the UE 10 and the receiver may be a part ofthe base station 20.

Meanwhile, the wireless communication system may be generally dividedinto a frequency division duplex (FDD) type and a time division duplex(TDD) type. According to the FDD type, uplink transmission and downlinktransmission are achieved while occupying different frequency bands.According to the TDD type, the uplink transmission and the downlinktransmission are achieved at different time while occupying the samefrequency band. A channel response of the TDD type is substantiallyreciprocal. This means that a downlink channel response and an uplinkchannel response are approximately the same as each other in a givenfrequency area. Accordingly, in the TDD based wireless communicationsystem, the downlink channel response may be acquired from the uplinkchannel response. In the TDD type, since an entire frequency band istime-divided in the uplink transmission and the downlink transmission,the downlink transmission by the base station and the uplinktransmission by the terminal may not be performed simultaneously. In theTDD system in which the uplink transmission and the downlinktransmission are divided by the unit of a subframe, the uplinktransmission and the downlink transmission are performed in differentsubframes.

Hereinafter, the LTE system will be described in detail.

FIG. 2 illustrates a structure of a radio frame according to FDD of 3rdgeneration partnership project (3GPP) long term evolution (LTE).

The radio frame of FIG. 2 may be found in the section 5 of 3GPP TS36.211 V10.4.0 (2011-12) “Evolved Universal Terrestrial Radio Access(E-UTRA); Physical Channels and Modulation (Release 10)”.

The radio frame includes 10 subframes indexed 0 to 9. One subframeincludes two consecutive slots. Accordingly, the radio frame includes 20slots. The time taken for one subframe to be transmitted is denoted TTI(transmission time interval). For example, the length of one subframemay be 1 ms, and the length of one slot may be 0.5 ms.

The structure of the radio frame is for exemplary purposes only, andthus the number of subframes included in the radio frame or the numberof slots included in the subframe may change variously.

Meanwhile, one slot may include a plurality of OFDM symbols. The numberof OFDM symbols included in one slot may vary depending on a cyclicprefix (CP).

FIG. 3 illustrates a structure of a downlink radio frame according toTDD in 3GPP LTE.

For this, 3GPP TS 36.211 V10.4.0 (2011-23) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8)”, Ch. 4 may be referenced, and this is for TDD (timedivision duplex).

Subframes having index #1 and index #6 are denoted special subframes,and include a DwPTS (Downlink Pilot Time Slot: DwPTS), a GP (GuardPeriod) and an UpPTS (Uplink Pilot Time Slot). The DwPTS is used forinitial cell search, synchronization, or channel estimation in aterminal. The UpPTS is used for channel estimation in the base stationand for establishing uplink transmission sync of the terminal. The GP isa period for removing interference that arises on uplink due to amulti-path delay of a downlink signal between uplink and downlink.

In TDD, a DL (downlink) subframe and a UL (Uplink) co-exist in one radioframe. Table 1 shows an example of configuration of a radio frame.

TABLE 1 UL-DL Switch-point Subframe index configuration periodicity 0 12 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U D 25 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms D S U U D DD D D D 5 10 ms D S U D D D D D D D 6 5 ms D S U U U D S U U D

‘D’ denotes a DL subframe, ‘U’ a UL subframe, and ‘S’ a specialsubframe. When receiving a UL-DL configuration from the base station,the terminal may be aware of whether a subframe is a DL subframe or a ULsubframe according to the configuration of the radio frame.

FIG. 4 illustrates an example of a resource grid for one uplink ordownlink slot in 3GPP LTE.

Referring to FIG. 4, the uplink slot includes a plurality of OFDM(orthogonal frequency division multiplexing) symbols in the time domainand NRB resource blocks (RBs) in the frequency domain. For example, inthe LTE system, the number of resource blocks (RBs), i.e., NRB, may beone from 6 to 110.

The resource block is a unit of resource allocation and includes aplurality of sub-carriers in the frequency domain. For example, if oneslot includes seven OFDM symbols in the time domain and the resourceblock includes 12 sub-carriers in the frequency domain, one resourceblock may include 7×12 resource elements (REs).

FIG. 5 illustrates a sturcture of a downlink subframe.

In FIG. 5, assuming the normal CP, one slot includes seven OFDM symbols,by way of example.

The DL (downlink) subframe is split into a control region and a dataregion in the time domain. The control region includes up to first threeOFDM symbols in the first slot of the subframe. However, the number ofOFDM symbols included in the control region may be changed. A PDCCH(physical downlink control channel) and other control channels areassigned to the control region, and a PDSCH is assigned to the dataregion.

The physical channels in 3GPP LTE may be classified into data channelssuch as PDSCH (physical downlink shared channel) and PUSCH (physicaluplink shared channel) and control channels such as PDCCH (physicaldownlink control channel), PCFICH (physical control format indicatorchannel), PHICH (physical hybrid-ARQ indicator channel) and PUCCH(physical uplink control channel).

The PCFICH transmitted in the first OFDM symbol of the subframe carriesCIF (control format indicator) regarding the number (i.e., size of thecontrol region) of OFDM symbols used for transmission of controlchannels in the subframe. The wireless device first receives the CIF onthe PCFICH and then monitors the PDCCH.

Unlike the PDCCH, the PCFICH is transmitted through a fixed PCFICHresource in the subframe without using blind decoding.

The PHICH carries an ACK (positive-acknowledgement)/NACK(negative-acknowledgement) signal for a UL HARQ (hybrid automatic repeatrequest). The ACK/NACK signal for UL (uplink) data on the PUSCHtransmitted by the wireless device is sent on the PHICH.

The PBCH (physical broadcast channel) is transmitted in the first fourOFDM symbols in the second slot of the first subframe of the radioframe. The PBCH carries system information necessary for the wirelessdevice to communicate with the base station, and the system informationtransmitted through the PBCH is denoted MIB (master information block).In comparison, system information transmitted on the PDSCH indicated bythe PDCCH is denoted SIB (system information block).

The PDCCH may carry activation of VoIP (voice over internet protocol)and a set of transmission power control commands for individual UEs insome UE group, resource allocation of an upper layer control messagesuch as a random access response transmitted on the PDSCH, systeminformation on DL-SCH, paging information on PCH, resource allocationinformation of UL-SCH (uplink shared channel), and resource allocationand transmission format of DL-SCH (downlink-shared channel). A pluralityof PDCCHs may be sent in the control region, and the terminal maymonitor the plurality of PDCCHs. The PDCCH is transmitted on one CCE(control channel element) or aggregation of some consecutive CCEs. TheCCE is a logical allocation unit used for providing a coding rate perradio channel's state to the PDCCH. The CCE corresponds to a pluralityof resource element groups. Depending on the relationship between thenumber of CCEs and coding rates provided by the CCEs, the format of thePDCCH and the possible number of PDCCHs are determined.

The control information transmitted through the PDCCH is denoteddownlink control information (DCI). The DCI may include resourceallocation of PDSCH (this is also referred to as DL (downlink) grant),resource allocation of PUSCH (this is also referred to as UL (uplink)grant), a set of transmission power control commands for individual UEsin some UE group, and/or activation of VoIP (Voice over InternetProtocol).

The base station determines a PDCCH format according to the DCI to besent to the terminal and adds a CRC (cyclic redundancy check) to controlinformation. The CRC is masked with a unique identifier (RNTI; radionetwork temporary identifier) depending on the owner or purpose of thePDCCH. In case the PDCCH is for a specific terminal, the terminal'sunique identifier, such as C-RNTI (cell-RNTI), may be masked to the CRC.Or, if the PDCCH is for a paging message, a paging indicator, forexample, P-RNTI (paging-RNTI) may be masked to the CRC. If the PDCCH isfor a system information block (SIB), a system information identifier,SI-RNTI (system information-RNTI), may be masked to the CRC. In order toindicate a random access response that is a response to the terminal'stransmission of a random access preamble, an RA-RNTI (randomaccess-RNTI) may be masked to the CRC.

In 3GPP LTE, blind decoding is used for detecting a PDCCH. The blinddecoding is a scheme of identifying whether a PDCCH is its own controlchannel by demasking a desired identifier to the CRC (cyclic redundancycheck) of a received PDCCH (this is referred to as candidate PDCCH) andchecking a CRC error. The base station determines a PDCCH formataccording to the DCI to be sent to the wireless device, then adds a CRCto the DCI, and masks a unique identifier (this is referred to as RNTI(radio network temporary identifier) to the CRC depending on the owneror purpose of the PDCCH.

A control region in a subframe includes a plurality of control channelelements (CCEs). The CCE is a logical allocation unit used to providethe PDCCH with a coding rate depending on a radio channel state, andcorresponds to a plurality of resource element groups (REGs). The REGincludes a plurality of resource elements. According to an associationrelation of the number of CCEs and the coding rate provided by the CCEs,a PDCCH format and the number of bits of an available PDCCH aredetermined.

One REG includes 4 REs. One CCE includes 9 REGs. The number of CCEs usedto configure one PDCCH may be selected from a set {1, 2, 4, 8}. Eachelement of the set {1, 2, 4, 8} is referred to as a CCE aggregationlevel.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a wireless device having agood DL channel state can use one CCE in PDCCH transmission. A wirelessdevice having a poor DL channel state can use 8 CCEs in PDCCHtransmission.

A control channel consisting of one or more CCEs performs interleavingon an REG basis, and is mapped to a physical resource after performingcyclic shift based on a cell identifier (ID).

Meanwhile, a UE is unable to know that the PDCCH of its own istransmitted on which position within control region and using which CCEaggregation level or DCI format. Since a plurality of PDCCHs may betransmitted in one subframe, the UE monitors a plurality of PDCCHs inevery subframe. Here, the monitoring is referred to try to decode thePDCCH by the UE according to the PDCCH format.

In 3GPP LTE, in order to decrease the load owing to the blind decoding,a search space is used. The search space may be referred to a monitoringset of CCE for the PDCCH. The UE monitors the PDCCH within thecorresponding search space.

When a UE monitors the PDCCH based on the C-RNTI, the DCI format and thesearch space which is to be monitored are determined according to thetransmission mode of the PDSCH. The table below represents an example ofthe PDCCH monitoring in which the C-RNTI is setup.

TABLE 2 Transmission Transmission mode of PDSCH according to mode DCIformat Search space PDCCH Transmission DCI format 1A Public service andterminal Single antenna port, port 0 mode 1 specific DCI format 1Terminal specific Single antenna port, port 0 Transmission DCI format 1APublic service and terminal Transmit diversity mode 2 specific DCIformat 1 Terminal specific Transmit diversity Transmission DCI format 1APublic service and terminal Transmit diversity mode 3 specific DCIformat 2A Terminal specific CDD (Cyclic Delay Diversity) or transmitdiversity Transmission DCI format 1A Public service and terminalTransmit diversity mode 4 specific DCI format 2 Terminal specificClosed-loop spatial multiplexing Transmission DCI format 1A Publicservice and terminal Transmit diversity mode 5 specific DCI format 1DTerminal specific MU-MIMO (Multi-user Multiple Input Multiple Output)Transmission DCI format 1A Public service and terminal Transmitdiversity mode 6 specific DCI format 1B Terminal specific Closed-loopspatial multiplexing Transmission DCI format 1A Public service andterminal If the number of PBCH transmisison ports is mode 7 specific 1,single antenna port, port 0. Otherwise, transmit diversity DCI format 1Terminal specific Single antenna port, port 5 Transmission DCI format 1APublic service and terminal If the number of PBCH transmisison ports ismode 8 specific 1, single antenna port, port 0. Otherwise, transmitdiversity DCI format 2B Terminal specific Dual layer transmisison (port7 or 8), or single antenna port, port 7 or 8 Transmission DCI format 1APublic service and terminal Non-MBSFN subframe: if the number of mode 9specific PBCH antenna ports is 1, port 0 is used as independent antennaport. Otherwise, transmit Diversity MBSFN subframe: port 7 asindependent antenna port DCI format 2C Terminal specific 8 transmisisonlayers, ports 7-14 are used or port 7 or 8 is used as independentantenna port Transmission DCI 1A Public service and terminal Non-MBSFNsubframe: if the number of mode 10 specific PBCH antenna ports is 1,port 0 is used as independent antenna port. Otherwise, transmitDiversity MBSFN subframe: port 7 as independent antenna port DCI format2D Terminal specific 8 transmisison layers, ports 7-14 are used or port7 or 8 is used as independent antenna port

The usage of the DCI format is classified as shown in Table 3 below.

TABLE 3 DCI format Contents DCI format 0 Used in PUSCH scheduling DCIformat 1 Used in scheduling of one PDSCH codeword DCI format 1A Used incompact scheduling of one PDSCH codeword and random access process DCIformat 1B Used in compact scheduling of one PDSCH codeword havingprecoding information DCI format 1C Used in very compact scheduling ofone PDSCH codeword DCI format 1D Used in precoding and compactscheduling of one PDSCH codeword having power offset information DCIformat 2 Used in PDSCH scheduling of terminals configured in closed-loopspatial multiplexing mode DCI format 2A Used in PDSCH scheduling ofterminals configured in open-loop spatial multiplexing mode DCI format2B DCI format 2B is used for resouce allocation for dual-layerbeam-forming of PDSCH. DCI format 2C DCI format 2C is used for resouceallocation for closed-loop SU-MIMO or MU- MIMO operation to 8 layers.DCI format 2D DCI format 2C is used for resouce allocation to 8 layers.DCI format 3 Used to transmit TPC command of PUCCH and PUSCH having 2bit power adjustments DCI format 3A Used to transmit TPC command ofPUCCH and PUSCH having 1 bit power adjustment DCI format 4 Used in PUSCHscheduling of uplink (UP) operated in multi-antenna port transmisisonmode

For example, DCI format 0 includes fields listed in the following tablewith reference to section 5.3.3.1.1 of 3GPP TS 36.212 V10.2.0 (2011-06)

TABLE 4 Number of Field bits Carrier indicator 0 or 3 bits Flag forformat0/format1A differentiation 1 bit FH (Frequency hopping) flag 1 bitResource block assignment and hopping resource allocation MCS(Modulation and coding scheme) and RV (redundancy 5 bits version) NDI(New data indicator) 1 bit TPC 2 bits Cyclic shift for DM RS and OCCindex 3 bits UL index 2 bits DAI(Downlink Assignment Index) 2 bit CSIrequest 1 or 2 bits SRS request 0 or 1 bits Resource allocation type 1bit

FIG. 6. illustrates a structure of an uplink subframe in 3GPP LTE.

Referring to FIG. 6, an uplink subframe may be divided into a controlregion and a data region in a frequency domain. The control region isallocated a PUCCH for transmission of uplink control information. Thedata region is allocated a PUSCH for transmission of data (along withcontrol information in some cases).

A PUCCH for one UE is allocated a RB pair in a subframe. RBs in the RBpair take up different subcarriers in each of first and second slots. Afrequency occupied by the RBs in the RB pair allocated to the PUCCHchanges with respect to a slot boundary, which is described as the RBpair allocated to the PUCCH having been frequency-hopped on the slotboundary.

A UE transmits uplink control information through different subcarriersaccording to time, thereby obtaining a frequency diversity gain. m is alocation index indicating the logical frequency-domain location of an RBpair allocated for a PUCCH in a subframe.

Uplink control information transmitted on a PUCCH may include a HARQACK/NACK, a channel quality indicator (CQI) indicating the state of adownlink channel, a scheduling request (SR) which is an uplink radioresource allocation request, or the like.

A PUSCH is mapped to a uplink shared channel (UL-SCH) as a transportchannel. Uplink data transmitted on a PUSCH may be a transport block asa data block for a UL-SCH transmitted during a TTI. The transport blockmay be user information. Alternatively, the uplink data may bemultiplexed data. The multiplexed data may be the transport block forthe UL-SCH multiplexed with control information. For example, controlinformation multiplexed with data may include a CQI, a precoding matrixindicator (PMI), an HARQ, a rank indicator (RI), or the like.Alternatively, the uplink data may include only control information.

A carrier aggregation system is described hereinafter.

A carrier aggregation system aggregates a plurality of componentcarriers (CCs). A conventional definition of a cell is changed accordingto carrier aggregation. According to carrier aggregation, a cell maydenote a combination of a downlink component carrier and an uplinkcomponent carrier or a downlink component carrier alone.

Further, in carrier aggregation, cells may be divided into a primarycell, a secondary cell, and a serving cell. A primary cell denotes acell operating at a primary frequency, in which a UE performs an initialconnection establishment procedure or a connection reestablishmentprocedure with a BS or which is designated as a primary cell in ahandover procedure. A secondary cell denotes a cell operating at asecondary frequency, which is configured once RRC connection isestablished and is used to provide an additional radio resource.

As described above, the carrier aggregation system may support aplurality of component carriers (CCs), that is, a plurality of servingcells, unlike a single carrier system.

The carrier aggregation system may support cross-carrier scheduling.Cross-carrier scheduling is a scheduling method for performing resourceallocation for a PDSCH transmitted through a different component carrierthrough a PDCCH transmitted through a specific component carrier and/orresource allocation for a PUSCH transmitted through a component carrierdifferent from a component carrier basically linked with the specificcomponent carrier.

FIG. 7 illustrates an example of comparison between a single carriersystem and a carrier aggregation system.

Referring to (a) of FIG. 7, the single carrier system supports only onecarrier for an uplink and a downlink for a UE. Although there may bevarious bandwidths of carriers, a UE is assigned one carrier. Referringto (b) of FIG. 7, the carrier aggregation (CA) system may assign aplurality of component carriers (DL CC A to C and UL CC A to C) for aUE. A component carrier (CC) denotes a carrier used in the carrieraggregation system and may be abbreviated to a carrier. For example,three 20-MHz component carriers may be assigned to allocate a 60-MHzbandwidth for the terminal.

Carrier aggregation systems may be divided into a contiguous carrieraggregation system in which aggregated carriers are contiguous and anon-contiguous carrier aggregation system in which aggregated carriersare spaced apart from each other. Hereinafter, when simply referring toa carrier aggregation system, it should be understood as including botha case where component carriers are contiguous and a case wherecomponent carriers are non-contiguous. Different numbers of componentcarriers may be aggregated for a downlink and an uplink. A case wherethe number of downlink component carriers and the number of uplinkcomponent carriers are the same is referred to as symmetric aggregation,and a case where the numbers are different is referred to as asymmetricaggregation.

When one or more component carriers are aggregated, component carriersto be aggregated may use the same bandwidths as adopted in an existingsystem for backward compatibility with the existing system. For example,the 3GPP LTE system supports bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10MHz, 15 MHz, and 20 MHz, and the 3GPP LTE-A system may configure a broadband of 20 MHz or more only using the bandwidths of the 3GPP LTE system.Alternatively, instead of using the bandwidths of the existing system,new bandwidths may be defined to configure a broad band.

In order to transmit/receive packet data through a specific secondarycell in carrier aggregation, a UE first needs to complete configurationfor the specific secondary cell. Here, configuration means thatreception of system information necessary for datatransmission/reception in a cell is completed. For example,configuration may include an overall process of receiving commonphysical-layer parameters necessary for data transmission and reception,media access control (MAC)-layer parameters, or parameters necessary fora specific operation in an RRC layer. A configuration-completed cell isin a state where packet transmission and reception is immediatelypossible upon receiving information indicating packet data may betransmitted.

A configuration-completed cell may be in an activated or deactivatedstate. Here, the activated state means that the cell performs datatransmission or reception or is ready for data transmission orreception. A UE may monitor or receive a control channel (PDCCH) and adata channel (PDSCH) of an activated cell in order to identify resources(which may be a frequency or time) assigned thereto.

The deactivated state means that transmission or reception of trafficdata is impossible and measurement or transmission/reception of minimalinformation is possible. A UE may receive system information (SI)necessary for receiving a packet from a deactivated cell. However, theUE does not monitor or receive a control channel (PDCCH) and datachannel (PDSCH) of the deactivated cell in order to identify resources(which may be a frequency or time) assigned thereto.

FIG. 8 illustrates cross-carrier scheduling in a carrier aggregationsystem.

Referring to FIG. 8, a BS may configure a PDCCH monitoring DL CC(monitoring CC) set. The PDCCH monitoring DL CC set includes some of allaggregated DL CCs, and a UE performs PDCCH monitoring/decoding only on aDL CC included in the PDCCH monitoring DL CC set when cross-carrierscheduling is configured. That is, the BS transmits a PDCCH with respectto a PDSCH/PUSCH to be scheduled only through a DL CC included in thePDCCH monitoring DL CC set. The PDCCH monitoring DL CC set may beconfigured to be UE-specific, UE group-specific, or cell-specific.

FIG. 8 illustrates an example in which three DL CCs (DL CC A, DL CC B,and DL CC C) are aggregated and DL CC A is set as a PDCCH monitoring DLCC. A UE may receive a DL grant with respect to a PDSCH of DL CC A, DLCC B, and DL CC C through a PDCCH of DL CC A. DCI transmitted throughthe PDCCH of DL CC A includes a CIF to indicate which DL CC the DCI isabout.

Hereinafter, a general random access procedure will be described. Arandom access procedure is used by a UE to achieve uplinksynchronization with a BS or to be assigned an uplink radio resource. Arandom access procedure may be divided into contention-based randomaccess and contention-free or non-contention-based random access.

FIG. 9a illustrates a contention-based random access procedure.

Referring to FIG. 9a , a UE 100 randomly selects one random accesspreamble in a random access preamble set indicated through systeminformation or a handover command. The UE 100 selects a radio resourcefor transmitting the random access preamble to transmit the selectedrandom access preamble (message 1: Msg 1, S1111). The radio resource maybe a specific subframe, and selecting the radio resource may beselecting a physical random access channel (PRACH).

After transmitting the random access preamble, the UE 100 attempts toreceive a random access response (RAR) within an RAR window indicatedthrough the system information or handover command and accordinglyreceives an RAR (message 2: Msg 2, S1112). The RAR may be transmitted ina MAC protocol data unit (PDU) format.

The RAR may include a random access preamble identifier (ID), a UL grant(uplink radio resource), a temporary cell-radio network temporaryidentifier (C-RNTI), and a synchronization adjustment command (forexample, a timing advance command (TAC)). Since one RAR may include RARinformation for one or more UEs 100, a random access preamble ID may beincluded to indicate a UE 100 for which a UL grant, a temporary C-RNTI,and a synchronization adjustment command (for example, a TAC) are valid.The random access preamble ID may be an ID of a random access preamblereceived by an eNodeB 200. The synchronization adjustment command (forexample, a TAC) may be included as information for the UE 100 to adjustuplink synchronization. The RAR may be indicated by a random access IDon a PDCCH, that is, a random access-radio network temporary identifier(RA-RNTI).

When the UE 100 receives the RAR valid therefor, the UE 100 processesinformation included in the RAR and performs scheduled transmission(message 3: Msg 3) to the eNodeB 200. That is, the UE 100 applies thesynchronization adjustment command (for example, a TAC) and stores thetemporary C-RNTI. Further, the UE 100 transmits data stored in a bufferof the UE 100 or newly generated data to the eNodeB 200 using the ULgrant. In this case, information to identify the UE 100 needs to beincluded, which is for identifying the UE 100 in order to avoidcontention since the eNodeB 200 does not judge which UEs 100 performrandom access in the contention-based random access procedure.

The UE 100 transmits a scheduled message (that is, Msg 3) including anID of the UE 100 through a radio resource assigned through the UL grantincluded in the RAR and waits for an instruction (message 4: Msg 4) fromthe eNodeB 200 to avoid contention (S1114). That is, the UE 100 attemptsto receive a PDCCH in order to a specific message.

FIG. 9b illustrates a non-contention-based random access procedure.

Unlike contention-based random access, non-contention-based randomaccess may be finished when a UE 100 receives an RAR.

Non-contention-based random access may be initiated by a request, suchas a handover and/or a command from an eNodeB 200. Here, in theforegoing two cases, contention-based random access may also beperformed.

The UE 100 is assigned by the eNodeB 200 a designated random accesspreamble having no possibility of contention. The random access preamblemay be assigned through a handover command and a PDCCH command (S1121).

After being assigned the random access preamble designated for the UE100, the UE 100 transmits the random access preamble to the eNodeB 200(S1122).

When the random access preamble us received, the eNodeB 200 transmits anRAR to the UE 100 in response (S1123).

Hereinafter, machine-type communication (MTC) will be described.

FIG. 10a illustrates an example of MTC.

MTC refers to an information exchange between MTC devices 100 via a BS200 or information exchange between an MTC device 100 and an MTC server700 via a BS without involving human interactions.

The MTC server 700 is an entity to communicate with the MTC device 100.The MTC server 700 runs an MTC application and provides the MTC devicewith an MTC-specific service.

The MTC device 100 is a wireless device to provide MTC communication,which may be stationary or mobile.

Services provided through MTC are differentiated from existingcommunication services involving human intervention and an MTC servicerange is wide, for example, tracking, metering, payment, medicalservices, remote control, or the like. More specifically, examples ofMTC services may include reading a meter, measuring a water level,utilizing a surveillance camera, inventory reporting of a vendingmachine, etc.

The MTC device is characterized in that a transmission data amount issmall and uplink/downlink data transmission/reception occurs sometimes.Therefore, it is effective to decrease a unit cost of the MTC device andto decrease battery consumption according to a low data transmissionrate. The MTC device is characterized by low mobility and thus has achannel environment that hardly changes.

FIG. 10b illustrates an example of cell coverage extension for an MTCdevice.

Recently, extension of cell coverage of a BS is considered for an MTCdevice 100, and various schemes for extending cell coverage are underdiscussion.

However, when the cell coverage is extended, if the BS transmits a PDSCHand a PDCCH including scheduling information on the PDSCH to an MTCdevice located in the coverage extension region as if transmitting thePDSCH and the PDCCH to a normal UE, the MTC device has difficulty inreceiving the PDSCH and the PDCCH.

Likewise, when the MTC device located in the coverage extension regiontransmits a physical random access channel (PRACH or a PRACH preamble)to the BS in a usual way, the BS may have difficulty in receiving thePRACH transmitted from the MTC device.

Thus, a first embodiment of the present invention is provided to solvethe foregoing problem.

According to the embodiment of the present invention, to solve theforegoing problem, when the MTC device 100 located in the coverageextension region transmits a PRACH to the BS, the MTC device 100repeatedly transmits a plurality of preambles according to a specificrepetition level. For example, when the MTC device 100 is located in aplace where signal reception is poor (for example, in the cell coverageextension region), such as under a bridge or in a basement, the MTCdevice 100 may repeatedly transmit a random access preamble in theembodiment of the present invention. Likewise, the BS may repeatedlytransmit an RAR (that is, Msg 2) to the random access preamble to theMTC device. Then, the MTC device 100 may repeatedly transmit a scheduledmessage (that is, Msg 3) based on the RAR. Further, the BS may alsorepeatedly transmit Msg 4. Meanwhile, when the MTC device 100 receivesno RAR from the BS even though repeatedly transmitting the random accesspreamble or when the MTC device 100 receives no Msg 4 from the BS eventhough repeatedly transmitting Msg 3, it may be technically unclearwhether the MTC device 100 needs to retransmit the random accesspreamble according to the same repetition level as the previous one orneeds to change the repetition level to retransmit the random accesspreamble.

Thus, a second embodiment of the present invention is provided to solveeven such a problem.

According to the second embodiment of the present invention, to solvethe foregoing problem, when the BS does not properly receive a PRACH,the MTC device retransmits the PRACH by performing power ramping orchanging the specific repetition level.

Hereinafter, embodiments of the present invention will sequentially bedescribed with reference to drawings.

I. First Embodiment of the Present Invention

A method of performing a random access procedure according to the firstembodiment of the present invention is a method of performing a randomaccess procedure by an MTC device located in a coverage extensionregion, which may include generating a random access preamble to aspecific cell and repeatedly transmitting the generated random accesspreamble according to a preset repetition level.

FIG. 11 illustrates a method of transmitting or retransmitting a PRACHaccording to an embodiment of the present invention.

Referring to FIG. 11, an MTC device 100 according to the firstembodiment of the present invention may transmit an initial PRACHpreamble to an eNodeB according to a specific repetition level (T100).

For example, the specific repetition level may be 3 as illustrated inFIG. 3.

When the eNodeB does not properly receive the initial PRACH preamble,the MTC device 100 may retransmit the PRACH preamble.

Here, the MTC device 100 may change the specific repetition level orperform power ramping to retransmit the PRACH.

As illustrated in FIG. 11, retransmission case 1 (T110) shows that onlythe repetition level is changed (from 3 to 4) to retransmit the PRACHpreamble, retransmission case 2 (T120) shows that only the number ofpower ramping times is changed (from 2 to 3) to retransmit the PRACHpreamble, and retransmission case 3 (T130) shows that both therepetition level and the number of power ramping times are changed toretransmit the PRACH preamble.

The PRACH repetition level may be changed when retransmission of thePRACH preamble is performed due to failure of RAR reception afterinitial transmission or failure of Msg 4 reception during contentionresolution.

In this case, increasing the repetition level may be considered alongwith power ramping for the PRACH.

Increasing the repetition level may denote increasing the number ofPRACH repetition times in PRACH retransmission.

According to an illustrative scheme for performing power ramping andincreasing the repetition level, it may be considered that therepetition level is increased after power ramping is performed to acertain level, or power ramping is performed after the repetition levelis changed.

Further, it may be considered to perform power ramping or change therepetition level using current total PRACH power as a parameter.

The total PRACH power may denote total power for a plurality ofsubframes corresponding to the repeated PRACH.

In a scheme for increasing the repetition level after power ramping isperformed to a certain level, the total number of (re)transmissions ofthe PRACH preamble may be configured according to first to third methodsas follows.

In a first method, the total number of (re)transmissions of the PRACHpreamble is expressed as a single parameter considering both powerramping and an increase in repetition level.

For example, parameter “preambleTransMax” denotes the total number of(re)transmissions of the PRACH preamble and may be configured in the MTCdevice through a higher-layer signal from the eNodeB.

Further, the number of power ramping times may be designated by eachrepetition level. For example, the number of power ramping times may bedesignated in advance or be configured in a high layer by eachrepetition level.

In a second method, the maximum number of power ramping times may beconfigured as a single parameter.

The parameter may be, for example, “preambleTransMax,” and the totalnumber of (re)transmissions of the PRACH preamble may actually beconfigured with “preambleTransMax” and a parameter representing arepetition level configured in the eNodeB according to an increase inrepetition level.

For example, the total number of (re)transmissions of the PRACH preamblemay be configured as “preambleTransMax*(# of repetition levels).”

Finally, in a third method, the maximum number of power ramping timesmay be configured by each repetition level.

A corresponding parameter may be expressed as preambleTransMax_m withrespect to repetition level m, and the total number of (re)transmissionsof the PRACH preamble may be expressed as the total maximum number ofpower ramping times with respect to repetition levels configured in theeNodeB.

II. Second Embodiment of the Present Invention

Meanwhile, PRACH retransmission may be performed in case A or case B asfollows.

Case A is a case where an RAR to a PRACH is not received in a configuredRAR window after initial transmission of the PRACH.

Case B is a case where a UE or the MTC device 100 does not receive Msg 4until a contention resolution timer configured in contention resolutionexpires.

When retransmission of the PRACH preamble is performed in case A or caseB, a PRACH repetition level may be configured the same or differentlyfor case A and case B.

Thus, a PRACH repetition level configuration method according to a firstaspect for case A (that is, where no RAR is received) and a PRACHrepetition level configuration method according to a second aspect forcase B (that is, where no Msg 4 is received) will be describedhereinafter.

First Aspect of Second Embodiment of the Present Invention

A method of performing a random access procedure according to the firstaspect is a method of performing a random access procedure for coverageenhancement, which may include repeatedly transmitting a random accesspreamble to a specific cell according to a preset repetition level,reconfiguring the repetition level when no RAR is received within an RARwindow, and repeatedly retransmitting the random access preambleaccording to the reconfigured repetition level.

The reconfiguring the repetition level may reconfigure the repetitionlevel based on a preset mode.

For example, the preset mode may be a mode of reconfiguring therepetition level to a possible lowest repetition level or a repetitionlevel selected based on a measurement by the MTC device.

Alternatively, the preset mode may be a mode of increasing the presetrepetition level by one.

Alternatively, the preset mode may be a mode of increasing the presetrepetition level by one when preamble transmission power P_(PRACH) isconfigured UE transmission power.

Alternatively, the preset mode may be a mode of increasing the presetrepetition level by one when preamble transmission power P_(PRACH) at atime of performing power ramping is configured UE transmission power orgreater.

That is, the preset mode may be a mode of increasing the presetrepetition level by one when transmission power for the random accesspreamble reaches maximum transmission power configured in the MTC deviceor the preamble transmission power at the time of performing powerramping is the maximum transmission power or greater.

Alternatively, the preset mode may be a mode of reconfiguring therepetition level based on total PRACH power, in which the total PRACHpower may be total power for subframes corresponding to the repeatedlytransmitted random access preamble.

Alternatively, the preset mode may be a mode of reconfiguring therepetition level according to repetition level information included in aPDCCH order when the random access procedure starts according to thePDCCH order.

FIG. 12 illustrates an example of the random access procedure in case Aaccording to the first aspect.

In the random access procedure according to the first embodiment of thepresent invention illustrated in FIG. 12, an MTC device 100 mayrepeatedly transmit a random access preamble (Msg 1) to a specific cellaccording to a preset repetition level (S110).

Next, an eNodeB 200 may transmit an RAR (Msg 2) to the random accesspreamble to the MTC device 100 (S1112).

When the MTC device 100 does not properly receive the RAR within an RARwindow (case A: failure of RAR reception within RAR window), the MTCdevice 100 may reconfigure the repetition level (S120).

Here, the MTC device 100 may reconfigure the repetition level based on apreset mode.

Next, the MTC device 100 may repeatedly retransmit the random accesspreamble according to the reconfigured repetition level (S130).

In case A, the preset mode may be one of the following modes.

Mode A-1

Mode A-1 may be a mode of changing a PRACH repetition level forretransmission based on a repetition level of an initially transmittedPRACH. That is, mode A-1 may be a mode of configuring the repetitionlevel to be an initial repetition level.

The initial repetition level may be a lowest repetition level or arepetition level selected based on a measurement by a UE or the MTCdevice 100.

That is, the preset mode may be a mode of reconfiguring the repetitionlevel to a possible lowest repetition level or a repetition levelselected based on a measurement by the MTC device.

Mode A-2

Mode A-2 may be a mode of simply increasing a repetition level forretransmitting a PRACH preamble.

For example, the MTC device 100 may increase a PRACH repetition levelfor retransmission by one.

That is, mode A-2 may be a mode of increasing the preset repetitionlevel by one.

Mode A-3

Mode A-3 may be a mode of configuring a repetition level in view ofPRACH power.

For example, the MTC device 100 may increase the repetition level by onewhen power for a previous PRACH is P_(CMAX,c)(i), while the MTC device100 may maintain the repetition level otherwise.

That is, the MTC device 100 may increase the preset repetition level byone when preamble transmission power P_(PRACH) is configured UEtransmission power.

Alternatively, the MTC device 100 may increase the repetition level byone when PRACH power at power ramping at the power for the previousPRACH is P_(CMAX,c)(i) or greater, while the MTC device 100 may maintainthe repetition level otherwise.

That is, the MTC device 100 may increase the preset repetition level byone when preamble transmission power P_(PRACH) at a time of performingpower ramping is configured UE transmission power or greater.

That is, the preset mode may be a mode of increasing the presetrepetition level by one when transmission power for the random accesspreamble reaches maximum transmission power configured in the MTC deviceor the preamble transmission power at the time of performing powerramping is the maximum transmission power or greater.

Alternatively, the MTC device 100 may determine whether to perform powerramping or to change a repetition value in view of total power for arepeated PRACH.

The total power may be a value obtained by adding up power per subframewith respect to subframes corresponding to the repeated PRACH.

That is, mode A-3 may be a mode of reconfiguring the repetition levelbased on total PRACH power, in which the total PRACH power may be totalpower for subframes corresponding to the repeatedly transmitted randomaccess preamble.

In this case, the MTC device 100 may select one involving smaller totalRACH power of power ramping at the power for the previous PRACH and achange of the repetition level.

That is, mode A-3 may be a mode of selecting and performing oneinvolving smaller total RACH power of power ramping and a change of thepreset repetition level, in which the total PRACH power may be totalpower for subframes corresponding to the repeatedly transmitted randomaccess preamble.

Mode A-4

Mode A-4 may be a mode of configuring a PRACH repetition level accordingto repetition level information included in a PDCCH order when a PRACHis configured according to the PDCCH order.

That is, mode A-4 may be a mode of reconfiguring a repetition levelaccording to repetition level information included in a PDCCH order whena random access procedure starts according to the PDCCH order.

Second Aspect of Second Embodiment of the Present Invention

A method of performing a random access procedure according to the secondaspect is a method of performing a random access procedure for coverageenhancement, which may include generating a random access preamble to aspecific cell, repeatedly transmitting the random access preambleaccording to a preset repetition level, transmitting a scheduled message(or message 3: Msg 3) when an RAR is received, reconfiguring the presetrepetition level when no message 4 (Msg 4) is received until acontention resolution timer expires, and repeatedly retransmitting therandom access preamble according to the reconfigured repetition level.

The reconfiguring the preset repetition level may reconfigure the presetrepetition level based on an additional preset mode.

For example, the additional preset mode may be a mode of maintaining thepreset repetition level in a case where the received RAR includes abackoff indicator, and increasing the preset repetition level in a casewhere the received RAR includes no backoff indicator.

Alternatively, the additional preset mode may be a mode of configuringthe preset repetition level to be a repetition level corresponding tothe scheduled message, in which the repetition level corresponding tothe scheduled message may be one of a possible lowest repetition level,a repetition level selected based on a measurement by the MTC device,and a repetition level corresponding to a random access preamblesuccessfully transmitted through retransmission before the scheduledmessage.

Alternatively, the received RAR includes a repetition level field, andthe additional preset mode may be a mode of reconfiguring the presetrepetition level according to the repetition level field.

Alternatively, the additional preset mode may be a mode of selecting andperforming one involving smaller total RACH power of power ramping and achange of the preset repetition level, in which the total PRACH powermay be total power for subframes corresponding to the repeatedlytransmitted random access preamble.

Alternatively, the additional preset mode may be a mode of reconfiguringthe preset repetition level according to repetition level informationincluded in a PDCCH order when the random access procedure startsaccording to the PDCCH order.

FIG. 13 illustrates an example of the random access procedure in case Baccording to the second aspect.

In the random access procedure according to the second aspect of thepresent invention illustrated in FIG. 13, an MTC device 100 mayrepeatedly transmit a random access preamble (Msg 1) to a specific cellaccording to a preset repetition level (S210).

Next, an eNodeB 200 may transmit an RAR (Msg 2) to the random accesspreamble to the MTC device 100 (S1112).

When an RAR valid for the MTC device 100 is received, the MTC device 100processes information included in the RAR and performs scheduledtransmission to the eNodeB 200 (S1113).

The MTC device 100 transmits data including an ID of the MTC device 100through a UL grant assigned via the received RAR and waits for aninstruction from the eNodeB 200 to avoid contention (S1114).

When the MTC device 100 does not properly receive Msg 4 until acontention resolution timer expires (case B: failure of Msg 4reception), the MTC device 100 may reconfigure the repetition level(S220).

Here, the MTC device 100 may reconfigure the repetition level based onan additional preset mode.

Next, the MTC device 100 may repeatedly retransmit the random accesspreamble according to the reconfigured repetition level (S230).

In case B, the additional preset mode may be one of the following modes.

Mode B-1

Mode B-1 may be a mode of configuring a PRACH repetition level dependingon the presence of a subheader of a backoff indicator in an RAR.

For example, the MTC device 100 may increase the PRACH repetition levelin the absence of the backoff indicator subheader in the RAR, andmaintain the PRACH repetition level in the presence of the backoffindicator subheader in the RAR.

That is, mode B-1 may be a mode of maintaining the preset repetitionlevel in a case where the received RAR includes a backoff indicator, andincreasing the preset repetition level in a case where the received RARincludes no backoff indicator.

Mode B-2

Mode B-2 may be a mode of changing a PRACH repetition level based on arepetition level of an initially transmitted PRACH. That is, mode A-1may be a mode of configuring the repetition level to be an initialrepetition level.

The initial repetition level may be a lowest repetition level or arepetition level selected based on a measurement by a UE or the MTCdevice 100.

That is, the additional preset mode may be a mode of reconfiguring therepetition level to a possible lowest repetition level or a repetitionlevel selected based on a measurement by the MTC device.

Mode B-3

Mode B-3 may be a mode of configuring a PRACH repetition levelcorresponding to a scheduled message (Msg 3) as a PRACH repetition levelfor retransmission.

The PRACH repetition level corresponding to the scheduled message may bea repetition level used for initial transmission or a repetition levelcorresponding to a PRACH successfully transmitted through retransmissionbefore the scheduled message is transmitted.

That is, mode B-3 may be a mode of configuring the preset repetitionlevel to be a repetition level corresponding to the scheduled message,in which the repetition level corresponding to the scheduled message maybe one of a possible lowest repetition level, a repetition levelselected based on a measurement by the MTC device, and a repetitionlevel corresponding to a random access preamble successfully transmittedthrough retransmission before the scheduled message.

Mode B-4

Mode B-4 may be a mode of configuring a PRACH repetition level forretransmission using a repetition level field, which is included in anRAR.

That is, according to mode B-4, the received RAR includes a repetitionlevel field, and the additional preset mode may be a mode ofreconfiguring the preset repetition level according to the repetitionlevel field.

Mode B-5

Mode B-5 may be a mode of simply increasing a repetition level forretransmitting a PRACH preamble.

For example, the MTC device 100 may increase a PRACH repetition levelfor retransmission by one.

That is, mode B-5 may be a mode of increasing the preset repetitionlevel by one.

Mode B-6

Mode B-6 may be a mode of maintaining a repetition level forretransmitting a PRACH preamble.

Mode B-7

Mode B-7 may be a mode of configuring a repetition level in view ofPRACH power

For example, the MTC device 100 may increase the repetition level by onewhen power for a previous PRACH is P_(CMAX,c)(i), while the MTC device100 may maintain the repetition level otherwise.

That is, the MTC device 100 may increase the preset repetition level byone when preamble transmission power P_(PRACH) is configured UEtransmission power.

Alternatively, the MTC device 100 may increase the repetition level byone when PRACH power at power ramping at the power for the previousPRACH is P_(CMAX,c)(i) or greater, while the MTC device 100 may maintainthe repetition level otherwise.

That is, the MTC device 100 may increase the preset repetition level byone when preamble transmission power P_(PRACH) at a time of performingpower ramping is configured UE transmission power or greater.

That is, the additional preset mode may be a mode of increasing thepreset repetition level by one when transmission power for the randomaccess preamble reaches maximum transmission power configured in the MTCdevice or the preamble transmission power at the time of performingpower ramping is the maximum transmission power or greater.Alternatively, the MTC device 100 may determine whether to perform powerramping or to change a repetition value in view of total power for arepeated PRACH.

The total power may be a value obtained by adding up power per subframewith respect to subframes corresponding to the repeated PRACH.

That is, mode B-7 may be a mode of reconfiguring the repetition levelbased on total PRACH power, in which the total PRACH power may be totalpower for subframes corresponding to the repeatedly transmitted randomaccess preamble.

In this case, the MTC device 100 may select one involving smaller totalRACH power of power ramping at the power for the previous PRACH and achange of the repetition level.

That is, mode B-7 may be a mode of selecting and performing oneinvolving smaller total RACH power of power ramping and a change of thepreset repetition level, in which the total PRACH power may be totalpower for subframes corresponding to the repeatedly transmitted randomaccess preamble.

Mode B-8

Mode B-8 may be a mode of configuring a PRACH repetition level accordingto repetition level information included in a PDCCH order when a PRACHis configured according to the PDCCH order.

That is, mode B-8 may be a mode of reconfiguring a repetition levelaccording to repetition level information included in a PDCCH order whena random access procedure starts according to the PDCCH order.

III. Additional Embodiments of the Present Invention

<Configuration of Backoff Delay>

Hereinafter, a method of configuring a backoff delay in accordance witha repetition level according to an additional embodiment of the presentinvention will be described.

In the existing 3GPP LTE Rel-10 system, a backoff delay parameter may beincluded in an RAR. When no backoff delay parameter is included in theRAR, a backoff delay value may be set to 0 for a UE or MTC device 100.

Also, a backoff delay value may be set to 0 for a PRACH according to aPDCCH order.

When the RAR includes the backoff delay parameter, a backoff delay isconfigured with the backoff delay parameter.

When the UE or MTC device 100, which receives no Msg4 until expirationof a contention resolution timer in a contention resolution process,retransmits a PRACH, the UE or MTC device 100 may select a random valuefrom 0 to the backoff delay parameter as a backoff delay and retransmitthe PRACH after a delay of the backoff delay value.

Although a PRACH occupies up to three subframes conventionally, a PRACHmay occupy a greater number of subframes in view of repetition of thePRACH. In this case, a conventional backoff delay configuration methodmay be less effective in preventing possible contention betweenrepeatedly transmitted PRACHs and thus needs to be efficiently changed.

That is, it may be a quite suitable method to set a repetition level asa parameter in configuring a backoff delay.

For example, when the UE or MTC device 100 selects a random value from 0to the backoff delay parameter as a backoff delay value, a final backoffdelay value may be determined by modifying the selected random valueaccording to a repetition level.

In detail, the final backoff delay value may be determined bymultiplying the random value by the number of repetition times.

Alternatively, the final backoff delay value may be determined bymultiplying a preset correction value by repetition level by the randomvalue or adding the preset correction value to the random value.

<Configuration of PRACH Repetition Level in Handover>

Hereinafter, a method of configuring a PRACH repetition level inhandover according to an additional embodiment of the present inventionwill be described.

An MTC device 100 may support mobility.

In this case, the MTC device 100 may be handed over from a currentserving cell or source cell to a target cell.

The MTC device 100 may need to synchronize to UL timing so that the MTCdevice 100 accesses the target cell to receive UE-specific DL channelsor to transmit UL channels.

In this case, a higher layer may notify the MTC device 100 of arepetition level for transmission of a PRACH preamble to the target cellduring handover, thus configuring a repetition level suitable forretransmission of the PRACH preamble.

Alternatively, the MTC device 100 may be provided with SFN offsetinformation against an SFN for the target cell or an SFN for the sourcecell during handover.

Alternatively, it may be assumed that the SFN for the target cell andthe SFN for the source cell are the same in handover of the MTC device100.

Here, the SFNs being the same may mean that when radio frames of thesame SFNs for the two cells overlap more than half, a time differencebetween frames i of the two cells may be interpreted as being within153600·Ts. Such interpretation may be possible since operations of theMTC device 100 including random access and (E)PDCCH repetition areperformed using an SFN as a parameter.

The aforementioned embodiments of the present invention can beimplemented through various means. For example, the embodiments of thepresent invention can be implemented in hardware, firmware, software,combinations thereof, etc. Details thereof will be described withreference to the drawing

FIG. 14 is a block diagram of a wireless communication system accordingto an embodiment of the present invention.

A BS 200 includes a processor 201, a memory 202, and a radio frequency(RF) unit 203. The memory 202 is coupled to the processor 201, andstores a variety of information for driving the processor 201. The RFunit 203 is coupled to the processor 201, and transmits and/or receivesa radio signal. The processor 201 implements the proposed functions,procedures, and/or methods. In the aforementioned embodiment, anoperation of the BS may be implemented by the processor 201.

An MTC device 100 includes a processor 101, a memory 102, and an RF unit103. The memory 102 is coupled to the processor 101, and stores avariety of information for driving the processor 101. The RF unit 103 iscoupled to the processor 101, and transmits and/or receives a radiosignal. The processor 101 implements the proposed functions, procedures,and/or methods.

The processor may include Application-specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

An MTC device according to one embodiment of the present invention is anMTC device performing a random access procedure for coverageenhancement, which may include a transceiver to repeatedly transmit arandom access preamble to a specific cell according to a presetrepetition level, and a processor to reconfigure the repetition levelwhen no RAR is received within an RAR window and to control thetransceiver to repeatedly retransmit the random access preambleaccording to the reconfigured repetition level.

The transceiver may transmit message 3 (Msg 3) when an RAR is received,and the processor may reconfigure the preset repetition level when nomessage 4 (Msg 4) is received until a contention resolution timerexpires, and control the transceiver to repeatedly retransmit the randomaccess preamble according to the reconfigured repetition level.

Here, the transceiver may correspond to the RF unit 103 or be configuredto include the RF unit 103.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

As described above, the embodiments of the present invention solve theforegoing problems of the conventional technology. More specifically,the embodiments of the present invention may improve receptionperformance and decoding performance of an MTC device located in acoverage extension region of an eNodeB with respect to the eNodeB,thereby implementing a random access procedure in an efficient andexcellent manner.

What is claimed is:
 1. A method of performing a random access procedure,the method performed by a wireless device located in a coverageextension region of a long term evolution (LTE) based cell andcomprising: repeatedly transmitting a random access preamble accordingto a first repetition level to a cell; based on that no random accessresponse (RAR) including a random access preamble identifiercorresponding to the transmitted random access preamble is receivedwithin a RAR window from the cell, performing a repetition level changefrom the first repetition level to a second repetition level and a powerramping, wherein the repetition level change is performed firstly andthen the power ramping is performed; and repeatedly retransmitting therandom access preamble based on the repetition level change to thesecond repetition level and based on the power ramping, wherein based onthat the random access preamble is transmitted according to a physicaldownlink control channel (PDCCH) order, the PDCCH order includesrepetition level information.
 2. The method of claim 1, wherein therandom access preamble is repeatedly transmitted and retransmitted on aplurality of subframes.
 3. The method of claim 1, further comprising:transmitting a scheduled message when the RAR is received; and based onthat a message in response to the scheduled message is not receiveduntil a contention resolution timer expires, repeatedly retransmittingthe random access preamble.
 4. The method of claim 1, wherein the secondrepetition level is higher than the first repetition level, and whereininformation on the first and second the repetition levels is receivedvia a higher layer signal.
 5. The method of claim 3, wherein the powerramping is performed until reaching a maximum transmission powerconfigured in the wireless device.
 6. The method of claim 1, wherein thechange of the repetition level is performed based on a total PRACHpower, and wherein the total PRACH power corresponds to a summation ofpower on subframes where the random access preamble is repeatedlytransmitted.
 7. The method of claim 1, wherein the RAR is repeatedlytransmitted from the cell.
 8. A wireless device located in a coverageextension region to perform a random access procedure for coverageenhancement of a long term evolution (LTE) based cell, the wirelessdevice comprising: a transceiver to repeatedly transmit a random accesspreamble to a cell according to a first repetition level; and aprocessor to perform a repetition level change from the first repetitionlevel to a second repetition level and a power ramping, based on that norandom access response (RAR) including a random access preambleidentifier corresponding to the transmitted random access preamble isreceived within a RAR window from the cell, wherein the repetition levelchange is performed firstly and then the power ramping is performed,wherein the processor repeatedly retransmits the random access preamblebased on the repetition level change to the second repetition level andbased on the power ramping, and wherein based on that the random accesspreamble is transmitted according to a physical downlink control channel(PDCCH) order, the PDCCH order includes repetition level information. 9.The wireless device of claim 8, wherein the random access preamble isrepeatedly transmitted and retransmitted on a plurality of subframes.10. The wireless device of claim 8, wherein the processor transmits ascheduled message when the RAR is received; and wherein based on that amessage in response to the scheduled message is not received until acontention resolution timer expires, the processor repeatedlyretransmits the random access preamble.
 11. The wireless device of claim8, wherein the second repetition level is higher than the firstrepetition level, and wherein information on the first and second therepetition levels is received via a higher layer signal.
 12. Thewireless device of claim 8, wherein the power ramping is performed untilreaching a maximum transmission power configured in the wireless device.13. The wireless device of claim 8, wherein the change of the repetitionlevel is performed based on a total PRACH power, and wherein the totalPRACH power corresponds to a summation of power on subframes where therandom access preamble is repeatedly transmitted.